Photoaffinity labeling of the nuclear Ah receptor from mouse Hepa 1c1c7 cells using 2,3,7,8-[³H]tetrachlorodibenzo-p-dioxin

The photoinduced formation of the covalently labeled cytosolic and nuclear aryl hydrocarbon (Ah) receptors was studied using 2,3,7,8-[³H]tetrachlorodibenzo-p-dioxin (TCDD) as the photoaffinity label. Irradiation of TCDD alone at wavelengths of >300 nm resulted in rapid degradation of this compound (t( 1/2 ) = 8 min). In a separate experiment, the unliganded cytosolic Ah receptor was only slowly inactivated (t( 1/2 ) = 48 min) using the >300 nm light source. Preliminary experiments with rat hepatic cytosol did not result in significant formation of specifically bound [³H]TCDD-protein covalent adducts which could be visualized by autoradiography. Irradiation of [³H]TCDD-nuclear Ah receptor complexes isolated from mouse Hepa 1c1c7 cells for 15 min gave approximately a 40% overall yield of the radiolabeled Ah receptor protein adduct. Denaturing sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the [³H]TCDD-nuclear Ah receptor photoadduct gave a single major radiolabeled protein with an apparent molecular size of 91 kDa. The chromatographic properties of the control (dark) and photolabeled nuclear Ah receptor complexes were comparable using Sephacryl S-300 and DNA-Sepharose columns. Velocity sedimentation of both the control (dark) and irradiated nuclear Ah receptor complexes gave specifically bound peaks which sedimented at 6.5 S. However, the trichloroacetic acid-precipitable (buffer-reconstituted) [³H]TCDD-nuclear Ah receptor photocovalent adduct was eluted from the Sephacryl S-300 column in the void volume and did not exhibit a specifically bound peak after velocity sedimentation analysis due to protein aggregate formation. In contrast, the elution profile of the aggregate of a DNA-Sepharose column was similar to that observed for the control (dark) and photolabeled complexes, which were eluted from the column with salt concentrations between 0.24 and 0.28 M. These photolabeling studies show that [³H]TCDD can act as a photoaffinity label for the Ah receptor and can be utilized as photoligand to probe further the structure and function of this protein.